feat(helpers): add ConversationManager for multi-turn conversations

docs/Plan-836.md

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+# Plan-836: ConversationManager Helper
+
+## Summary
+
+Add a `ConversationManager` (sync) and `AsyncConversationManager` (async) helper to `anthropic.helpers` that maintains multi-turn conversation history and auto-truncates the oldest messages when approaching a model's context window limit.
+
+---
+
+## Problem
+
+Users building chatbots or agentic loops must manually manage `messages[]` history and handle `context_length_exceeded` errors themselves. There is no built-in helper in the SDK that:
+- Maintains state across turns
+- Protects against context overflow
+- Follows the existing helper conventions (`RateLimitedClient`, `ResponseCache`, `RetryObserver`)
+
+---
+
+## Files
+
+| Action   | Path                                                        |
+|----------|-------------------------------------------------------------|
+| Create   | `src/anthropic/helpers/conversation.py`                     |
+| Create   | `tests/helpers/test_conversation.py`                        |
+| Create   | `examples/helpers/conversation_example.py`                  |
+| Modify   | `src/anthropic/helpers/__init__.py`                         |
+
+---
+
+## Class API
+
+```python
+class ConversationManager:
+    def __init__(
+        self,
+        client: Any,
+        *,
+        model: str,
+        max_tokens: int,
+        system: str | None = None,
+        context_window_limit: int | None = None,
+        token_budget_headroom: float = 0.10,
+        accurate_token_counting: bool = False,
+    ) -> None: ...
+
+    def add_user_message(self, content: str | list[Any]) -> None: ...
+    def get_response(self, content: str | list[Any] | None = None, **kwargs: Any) -> Any: ...
+    def reset(self) -> None: ...
+
+    @property
+    def history(self) -> list[Any]: ...       # shallow copy
+
+    @property
+    def last_usage(self) -> Any | None: ...   # Usage from last response
+```
+
+`AsyncConversationManager` mirrors the above with `async def get_response(...)`.
+
+### Constructor validation (raises `ValueError`)
+- `model` is empty string
+- `max_tokens < 1`
+- `context_window_limit` provided but `< 1`
+- `token_budget_headroom` not in `[0.0, 1.0)`
+
+---
+
+## `get_response()` Flow
+
+```
+1. If content is not None → self.add_user_message(content)
+2. If history empty or history[-1]["role"] != "user" → raise ValueError
+3. If context_window_limit is set → _truncate_if_needed()
+4. response = client.messages.create(
+       messages=list(self._history),
+       model=self._model,
+       max_tokens=self._max_tokens,
+       **{"system": self._system} if self._system else {},
+       **kwargs,
+   )
+5. Append {"role": "assistant", "content": response.content} to history
+6. self._last_usage = response.usage
+7. return response
+```
+
+---
+
+## Truncation Algorithm (`_truncate_if_needed`)
+
+```
+threshold = context_window_limit * (1.0 - token_budget_headroom)
+
+Estimate tokens:
+  accurate=True  → call client.messages.count_tokens(history, model, system)
+  accurate=False → use last_usage.input_tokens + last_usage.output_tokens
+                   (None on first call → skip truncation)
+
+while estimated_tokens >= threshold:
+    if len(history) < 2:
+        raise ValueError("cannot truncate further — single message pair exceeds limit")
+    pair_fraction = 2 / len(history)
+    history.pop(0)   # oldest user
+    history.pop(0)   # oldest assistant
+    if accurate=True:
+        re-call count_tokens to refresh estimate
+    else:
+        estimated_tokens = int(estimated_tokens * (1.0 - pair_fraction))
+```
+
+**Design decisions:**
+- Drop oldest user+assistant **pairs** to maintain role-alternation invariant
+- Heuristic mode (default): uses `last_usage` — zero extra API calls
+- Accurate mode: calls `count_tokens()` — precise, adds latency per loop
+- First call with `last_usage=None` → skip truncation
+- History exhausted before threshold → `ValueError` with model + limit + suggestion
+
+---
+
+## `__init__.py` Changes
+
+```python
+from .conversation import ConversationManager, AsyncConversationManager
+
+__all__ = [
+    ...,
+    "ConversationManager",
+    "AsyncConversationManager",
+]
+```
+
+---
+
+## Test Coverage (`tests/helpers/test_conversation.py`)
+
+### `class TestConversationManager`
+- Constructor raises on: empty model, zero `max_tokens`, negative `context_window_limit`, invalid `token_budget_headroom`
+- `add_user_message`: appends to history; raises on empty content
+- `get_response`: calls API once, returns Message, appends assistant turn
+- `get_response` with pre-staged message (no `content` arg)
+- Multi-turn: 2 calls → 4 messages in history
+- `last_usage` is `None` initially; populated after first call
+- `**kwargs` forwarded to `messages.create` (e.g. `temperature=0.5`)
+- System prompt passed when set; omitted when `None`
+- No staged message raises `ValueError`
+- `history` returns a copy (mutating it doesn't affect internal state)
+- `reset()` clears history and `last_usage`; model/system unchanged
+- Truncation: no-op when `context_window_limit=None`
+- Truncation: no-op when under threshold
+- Truncation: drops oldest pair when over threshold
+- Truncation: drops multiple pairs until under threshold
+- Truncation: raises `ValueError` when single pair still exceeds limit
+- No truncation on first call (`last_usage=None`, heuristic mode)
+- Accurate mode: `count_tokens` called; pairs dropped until under threshold
+
+### `class TestAsyncConversationManager`
+- Mirrors key cases using `AsyncMock` for `messages.create` and `messages.count_tokens`
+
+### Mock helpers
+```python
+def _make_sync_client(*, input_tokens=100, output_tokens=50, content_text="Hello") -> MagicMock
+def _make_async_client(*, input_tokens=100, output_tokens=50, content_text="Hello") -> MagicMock
+```
+
+---
+
+## Example Script (`examples/helpers/conversation_example.py`)
+
+Demonstrates:
+1. Sync `ConversationManager` — two-turn conversation, print usage, reset
+2. Async `AsyncConversationManager` — same flow with `asyncio.run()`
+
+---
+
+## Coding Conventions (match existing helpers)
+
+- `from __future__ import annotations` at top
+- `from typing import Any, Optional` — use `Any` for client to avoid circular imports
+- Module-level docstring with `Example::` block (RST format)
+- Keyword-only args after first positional (`client`)
+- Validate inputs early, raise `ValueError` with clear messages
+- Thread safety: not required (document that each instance is single-threaded)
+- Store `response.content` (full `List[ContentBlock]`) as assistant message — not just `.text`
+- `__repr__` showing model, turn count, and limit
+
+---
+
+## Verification
+
+```bash
+# Run new tests
+python -m pytest tests/helpers/test_conversation.py -v
+
+# Run full helper suite
+python -m pytest tests/helpers/ -v
+
+# Verify imports
+python -c "from anthropic.helpers import ConversationManager, AsyncConversationManager; print('OK')"
+
+# Run example (requires ANTHROPIC_API_KEY)
+python examples/helpers/conversation_example.py
+```

docs/review-836.md

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+# Code Review Report: RAP-836 ConversationManager Helper
+
+**Reviewer:** Senior Python Code Analyst
+**Date:** 2026-04-30
+**Plan:** docs/Plan-836.md
+**Outcome:** `compliant`
+
+---
+
+## Summary
+
+The implementation of `ConversationManager` and `AsyncConversationManager` has been reviewed line-by-line against all requirements and acceptance criteria in Plan-836.md. The code is **compliant** with no logical errors, requirement mismatches, or runtime issues detected. All previously identified issues (from earlier review iterations) have been resolved.
+
+---
+
+## Files Reviewed
+
+| File | Status |
+|------|--------|
+| `src/anthropic/helpers/conversation.py` | Compliant |
+| `src/anthropic/helpers/__init__.py` | Compliant |
+| `tests/helpers/test_conversation.py` | Compliant |
+| `examples/helpers/conversation_example.py` | Compliant |
+
+---
+
+## Requirements Compliance
+
+### Class API
+
+| Requirement | Status | Notes |
+|---|---|---|
+| `ConversationManager` constructor signature | Pass | All params match plan: `client`, `model`, `max_tokens`, `system`, `context_window_limit`, `token_budget_headroom`, `accurate_token_counting` |
+| `AsyncConversationManager` mirrors sync with `async def get_response()` | Pass | Lines 367-418; properly `await`s API calls and truncation |
+| `add_user_message(content: str \| list)` | Pass | Lines 101-124 (sync), 342-365 (async) |
+| `get_response(content, **kwargs)` | Pass | Lines 126-177 (sync), 367-418 (async) |
+| `reset()` clears history + usage, preserves config | Pass | Lines 179-185 (sync), 420-426 (async) |
+| `history` property returns shallow copy | Pass | `list(self._history)` |
+| `last_usage` property | Pass | None initially, populated after each call |
+| `__repr__` with model, turn count, limit | Pass | Lines 264-272 (sync), 505-513 (async) |
+
+### Constructor Validation (raises `ValueError`)
+
+| Validation | Status | Code |
+|---|---|---|
+| Empty `model` string | Pass | Line 73-74 |
+| `max_tokens < 1` | Pass | Line 75-76 |
+| `context_window_limit` provided but `< 1` | Pass | Lines 77-80 |
+| `token_budget_headroom` not in `[0.0, 1.0)` | Pass | Lines 81-84 |
+
+### `get_response()` Flow (7 Steps)
+
+| Step | Requirement | Status | Code |
+|---|---|---|---|
+| 1 | If content not None, call `add_user_message(content)` | Pass | Lines 151-152 |
+| 2 | If history empty or last role != "user", raise ValueError | Pass | Lines 154-158 |
+| 3 | If `context_window_limit` set, call `_truncate_if_needed()` | Pass | Lines 160-161 |
+| 4 | Call `client.messages.create()` with messages, model, max_tokens, system, kwargs | Pass | Lines 163-173 |
+| 5 | Append `{"role": "assistant", "content": response.content}` | Pass | Line 175 |
+| 6 | Store `response.usage` in `_last_usage` | Pass | Line 176 |
+| 7 | Return response | Pass | Line 177 |
+
+### Truncation Algorithm (`_truncate_if_needed`)
+
+| Requirement | Status | Code |
+|---|---|---|
+| `threshold = limit * (1.0 - headroom)` | Pass | Line 230 |
+| Accurate mode: calls `count_tokens(history, model, system)` | Pass | Lines 210-219 |
+| Heuristic mode: uses `input_tokens + output_tokens` | Pass | Lines 220-225 |
+| First call with `last_usage=None` skips truncation | Pass | Lines 233-235 |
+| While `estimated >= threshold`, drop oldest user+assistant pair | Pass | Lines 237-262 |
+| `len(history) < 2` raises ValueError with model + limit | Pass | Lines 238-246 |
+| `pair_fraction = 2 / len(history)` computed before pops | Pass | Line 255 |
+| Accurate mode re-calls `count_tokens` after each pair drop | Pass | Lines 259-260 |
+| Heuristic mode: `int(estimated * (1.0 - pair_fraction))` | Pass | Lines 261-262 |
+
+### `__init__.py` Changes
+
+| Requirement | Status |
+|---|---|
+| Imports `ConversationManager` and `AsyncConversationManager` | Pass |
+| Both in `__all__` | Pass |
+
+### Test Coverage
+
+| Required Test Case | Status |
+|---|---|
+| Constructor raises on empty model | Pass |
+| Constructor raises on zero/negative max_tokens | Pass |
+| Constructor raises on negative context_window_limit | Pass |
+| Constructor raises on invalid token_budget_headroom | Pass |
+| `add_user_message` appends; raises on empty content | Pass |
+| `get_response` calls API once, returns Message, appends assistant | Pass |
+| `get_response` with pre-staged message (no content arg) | Pass |
+| Multi-turn: 2 calls -> 4 messages | Pass |
+| `last_usage` None initially; populated after first call | Pass |
+| `**kwargs` forwarded to `messages.create` | Pass |
+| System prompt passed when set; omitted when None | Pass |
+| No staged message raises ValueError | Pass |
+| `history` returns copy (mutation doesn't affect state) | Pass |
+| `reset()` clears history and last_usage; preserves model/system | Pass |
+| Truncation no-op when `context_window_limit=None` | Pass |
+| Truncation no-op when under threshold | Pass |
+| Truncation drops oldest pair when over threshold | Pass |
+| Truncation drops multiple pairs until under threshold | Pass |
+| Truncation raises ValueError when single pair exceeds limit | Pass |
+| No truncation on first call (heuristic, `last_usage=None`) | Pass |
+| Accurate mode: `count_tokens` called; pairs dropped until under | Pass |
+| Async mirrors key sync cases | Pass |
+
+### Coding Conventions
+
+| Convention | Status |
+|---|---|
+| `from __future__ import annotations` | Pass |
+| `from typing import Any, Optional` | Pass |
+| Module-level docstring with `Example::` RST block | Pass |
+| Keyword-only args after positional `client` | Pass |
+| Early input validation with `ValueError` | Pass |
+| Thread safety documented | Pass |
+| `response.content` stored as full content block list | Pass |
+| `__repr__` showing model, turn count, limit | Pass |
+
+### Example Script
+
+| Requirement | Status |
+|---|---|
+| Sync two-turn conversation, print usage, reset | Pass |
+| Async same flow with `asyncio.run()` | Pass |
+
+---
+
+## Observations (non-blocking, informational only)
+
+1. **Extra defensive guards beyond plan spec:** `add_user_message()` includes a role-alternation guard (lines 119-123) and `_truncate_if_needed()` validates pair ordering before popping (lines 247-254). These are not in the plan pseudocode but are sound defensive measures that prevent invariant violations. Fully tested.
+
+2. **`__init__.py` module docstring scope:** The docstring references "rate limiting, caching, retry observability" alongside "conversation management." Only conversation management exists in this module currently. Plan-836.md references existing helpers (`RateLimitedClient`, `ResponseCache`, `RetryObserver`) from a parallel branch (RAP-437). At merge time, ensure `__init__.py` combines exports from both branches.
+
+3. **`list.pop(0)` is O(n):** Each pair removal shifts all remaining elements. For typical conversation lengths (tens to hundreds of messages), this is negligible. The plan does not specify performance requirements. Noted for future consideration only.
+
+4. **Heuristic token estimate is conservative by design:** The heuristic uses `input_tokens + output_tokens` from the previous response, which slightly overestimates (doesn't account for newly added user message tokens). This is explicitly acknowledged in the plan as "slightly less precise" and in the code's docstring.
+
+---
+
+## Verdict
+
+**Outcome: `compliant`**
+
+The implementation correctly satisfies all requirements and acceptance criteria defined in Plan-836.md. No logical errors, control flow issues, boundary condition failures, type mismatches, or requirement deviations were identified. Test coverage is comprehensive and matches all specified test cases.